1. Field of the Invention
The invention relates to lifting screw mechanisms and, more particularly, the invention relates to throttle valve control assemblies for semiconductor wafer processing chambers.
2. Description of the Background Art
Throttle valves are used in many applications for adjusting the rate of flow of gases and liquids. A typical throttle valve regulates the flow of a liquid or gas by adjusting the position of a plunger relative to a valve housing. A particular application of throttle valves is in regulating the flow of gases in a semiconductor processing chamber. In such a throttle valve, linear movement of the plunger relative to the valve housing is accomplished by a lift screw mechanism. The lift screw mechanism transfers rotational movement of a threaded screw into linear movement of the plunger.
FIG. 1 depicts a simplified, partial cross-sectional view of a prior art throttle valve assembly 50 that employs a lift screw mechanism 103 coupled to a throttle valve 100. The throttle valve 100 comprises a valve housing 102 containing a plunger 104. The plunger 104 moves linearly relative to the valve housing 102 to regulate the flow of a liquid or gas from an inlet (not shown) through the housing 102 to an outlet (not shown). In the prior art throttle valve assembly 50, a hollow tube 107 extends from the plunger 104. A threaded nut 105 is fixed to an end of the tube 107. The nut 105 receives a threaded drive screw 106 with the end of the drive screw 106 extending into the tube 107. The drive screw 106 is coaxially aligned with an axis 101. A bearing 108 is affixed to the housing 102. The rive screw 106 rotates freely in the bearing 108. As the rive screw 106 rotates about the axis 101 the threads of the drive screw engage the threads of the nut 105 thereby imparting a linear motion to the plunger 104 along the axis 101. As the plunger 104 moves upward, the drive screw 106 is received in the tube 107.
Motive power for rotating the screw 106 is provided by a motor 118 such as an electric motor. A first pulley 110 is attached to a distal end of the drive screw 106. A drive belt 112 is looped about the first pulley 110 and a second pulley 114. The second pulley 114 is attached to a drive shaft 116 of the motor 118. The drive shaft 116 is aligned with a drive axis 55 that is substantially parallel to the axis 101. The motor 118 is fixed relative to the valve 100 such that rotation of the drive shaft 116 imparts motive power to the drive screw 106 via the pulleys 110 and 114 and the drive belt 112.
In the prior art throttle valve assembly 100, the axes 101 and 55 must be precisely aligned. A misalignment of the axes 101 and 55 (i.e., the axes are not parallel) will cause the drive shaft 116 to exert a bending stress on the drive screw 106. The bending stress causes a bending strain on the drive screw leading to wear that seriously shortens its useful life.
Therefore, a need exists in the art for a lift screw mechanism for a throttle valve assembly in which the drive screw is not required to be exactly parallel with the drive axis.